CN111526965B

CN111526965B - Method for butt-welding two metal plates by using first and second front laser beams and rear laser beam

Info

Publication number: CN111526965B
Application number: CN201880083419.4A
Authority: CN
Inventors: 勒内·维耶施特雷特
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2017-12-26
Filing date: 2018-12-19
Publication date: 2021-12-31
Anticipated expiration: 2038-12-19
Also published as: WO2019130169A1; WO2019130043A1; BR112020010179A2; MA51448A; JP2021509360A; KR20200087248A; RU2739265C1; US20210053152A1; CN111526965A; CA3086970A1; US11806808B2; HUE056639T2; MA51448B1; UA124997C2; CA3086970C; ZA202003549B; MX2020006707A; EP3731992B1; JP7118156B2; ES2901433T3

Abstract

A method of butt laser welding two metal sheets (2, 4), comprising: -providing a first metal sheet (2) and a second metal sheet (4), -butt-welding said metal sheets (2, 4) in a welding direction, the butt-welding step comprising simultaneous emission: a first front laser beam (12), said first front laser beam (12) producing a first front spot (18) at the intersection with the first metal plate (2) and producing a first front keyway in the first metal plate (2) at the first front spot (18); a second front laser beam (14), the second front laser beam (14) producing a second front spot (20) at the intersection with the second metal plate (4) and producing a second front keyway in the second metal plate (4) at the second front spot (20); a rear laser beam (16), the rear laser beam (16) producing a rear spot (22) on the first and second metal plates (2, 4) and a rear keyway in the first and second metal plates (2, 4) at the rear spot (22), the first and second front laser beams (12, 14) and the rear laser beam (16) being configured in such a way that at each time a solid phase region and/or a liquid phase region of the metal plates (2, 4) remains between the first front keyway and the rear keyway and between the second front keyway and the rear keyway.

Description

Method for butt-welding two metal plates by using first and second front laser beams and rear laser beam

The invention relates to a method for butt-welding two metal sheets.

Metal sheets for welding are typically obtained by cutting from a metal strip or larger metal sheet using cutting methods such as slitting, shearing, pressing, laser cutting or water jet cutting.

These cutting methods generally produce edge profiles with a relief or clearance angle, which creates a gap between the metal sheets when the metal sheets are arranged edge to edge in view of butt welding. The gap may result in no contact over the entire edge length over at least a portion of the thickness of the metal plate, or only at some point at the opposite edge of the metal plate. Such an initial gap may be further widened due to deformation of the metal plates caused by thermal stress during the welding process itself.

In some cases, it may also be desirable to apply and maintain a minimum weld gap between the plates, for example, if weld material is to be added to the weld puddle. In this case, the welding gap typically extends over the entire thickness and length of the opposite edges of the metal sheets, so that there is no contact between the two metal sheets to be welded to one another.

The inventors of the present invention have found that conventional laser welding methods using standard laser beams do not completely satisfactorily butt weld such metal plates together due to the presence of such gaps between the metal plates. Indeed, a significant portion of the laser beam energy is wasted as it passes through the gap and thus does not interact with the plate. In fact, the inventors have observed that, in general, only 10% to 20% of the laser beam energy is actually used to weld the plates, while the remaining 80% to 90% is wasted.

WO 2017/103149 relates to the problem of obtaining a welded joint with uniform material properties between two metal sheets having a zinc alloy or aluminium alloy pre-coating. To this end, WO 2017/103149 discloses a method for butt laser welding two such metal sheets using a filler wire and three laser beams, a first laser beam being intended for melting the filler wire and the other two laser beams being intended for melting the metal sheets and mixing the resulting weld pools. As can be seen from fig. 1c, the three laser beams cooperate to form a single weld pool using the Gibbs-Marangoni effect, so that a good mixing of the materials is obtained in the single weld pool. However, this method is not entirely satisfactory. In particular, it has relatively low energy efficiency and is therefore not suitable for welding two metal plates with a gap between them.

Laser brazing is also known for joining two metal sheets together. However, this joining method is not suitable for obtaining mechanical properties at least equal to those of the substrate in the joining zone.

One of the objects of the present invention is to overcome the above-mentioned drawbacks by proposing a method for butt-laser welding two metal sheets arranged edge to edge, thus obtaining an improved quality of the final product.

To this end, the invention relates to a method for butt laser welding two metal sheets, comprising the steps of:

-providing a first metal plate and a second metal plate, each metal plate having two main surfaces and a side surface joining the two main surfaces, respectively;

-positioning the first and second metal plates so that their side surfaces face each other, the positioning of the first and second metal plates defining a median plane perpendicular to the main surfaces of the first and second metal plates; and

-butt welding the first metal sheet and the second metal sheet along a welding direction, the butt welding step comprising simultaneously emitting:

-a first front laser beam along a first front emission axis intersecting one of the main surfaces of the first metal sheet, the first front laser beam generating a first front spot at the intersection with said main surface of the first metal sheet, the energy density of the first front laser beam being greater than or equal to 10⁶W/cm²A first front laser beam creates a first front keyway in the first metal plate at a first front spot;

-a second front laser beam along a second front emission axis intersecting one of the main surfaces of the second metal plate, the second front laser beam generating a second front spot at the intersection with said main surface of the second metal plate, the energy density of the second front laser beam being greater than or equal to 10⁶W/cm²A second front laser beam creates a second front keyhole in the second metal plate at a second front spot;

the respective centers of the first and second front spots are located at a distance of less than or equal to 2.5mm from the side surfaces of the first and second metal plates, respectively, and the distance between the centers of the first and second front laser beams taken in the welding direction is less than or equal to 5 mm; and

-a back laser beam intersecting the adjacent major surfaces of the first and second metal plates and producing a back spot thereon, the back laser beam having an energy density of greater than or equal to 10⁶W/cm²A rear spot having a surface area greater than a surface area of each of the first and second front spots, the rear laser beam creating a rear keyway in the first and second metal plates at the rear spot;

the first and second front and rear laser beams are configured in such a way that:

-the first front spot and the second front spot are located in front of the rear spot; and such that:

at each instant, a solid phase region and/or a liquid phase region of the metal sheet remains between the first front keyway and the rear keyway and between the second front keyway and the rear keyway.

According to a particular embodiment, the method according to the invention may also comprise one or more of the following features:

-at each instant of the butt-welding step, the volume of the weld pool produced by the first and second front laser beams is separated from the volume of the weld pool produced by the rear laser beam;

-the maximum size of the first front spot and/or the second front spot is between 50 μm and 250 μm;

the maximum size of the rear spot is from 200 μm to 1800 μm, preferably from 600 μm to 1200 μm;

-the first and second metal plates each have a thickness of 0.15mm to 5 mm;

the centers of the first and second front spots are located at equal distances from a mid-plane between the first and second metal plates;

-the centers of the first and second front spots are aligned in a direction perpendicular to the welding direction;

the centers of the first and second front spots are arranged at a distance from each other in the welding direction;

-the rear spot is centered on a mid-plane between the first metal plate and the second metal plate;

-the center of the rear spot is laterally offset with respect to a mid-plane between the first metal plate and the second metal plate;

-the centre of the rear spot extends at a distance of 0.5 to 8mm, preferably at a distance of 1 to 5mm, taken in the welding direction from the centre of the rearmost of the first and second front spots;

the first anterior spot and/or the second anterior spot has a gaussian or top hat (top hat) energy distribution and preferably a circular profile;

-the posterior spot has a gaussian or top-hat energy distribution;

the posterior spot is annular;

-the outer dimension of the rear spot, taken perpendicular to the welding direction, is smaller than the outer dimension of the rear spot, taken parallel to the welding direction;

the rear spot is symmetrical with respect to a plane parallel to the median plane between the two metal plates;

the maximum outer dimension of the rear spot is from 200 μm to 1800 μm, preferably from 600 μm to 1200 μm;

-the ratio of the maximum outer dimension to the maximum inner dimension of the rear spot is between 1.2 and 3.2, preferably between 1.3 and 2;

the rear spot has a circular profile or an elongated shape along an elongation direction parallel to the welding direction;

-the method further comprises: emitting a second back laser beam simultaneously with the steps of emitting the first front laser beam, the second front laser beam and the back laser beam, the second back laser beam intersecting the adjacent major surfaces of the first metal plate and the second metal plate and creating a second back spot 36 thereon, the second back laser beam being configured in such a way that the second back spot is located behind the back spot;

the second rear spot is annular or has a gaussian or top-hat energy distribution;

-the maximum outer dimension of the rear spot is larger than the maximum outer dimension of the second rear spot;

-the method further comprises providing a welding material, such as welding wire or welding powder, during the butt welding step;

-the first metal sheet and/or the second metal sheet comprises a steel substrate having a zinc alloy or aluminium alloy pre-coating on at least one of its main surfaces;

the first and/or second front and/or rear laser beams are generated by a common laser head;

-each laser beam is generated by a dedicated laser head;

-the steel substrate of at least one of the first metal sheet or the second metal sheet is a press hardenable steel; and the number of the first and second groups,

-at least one of the first metal sheet or the second metal sheet comprises a pre-coating comprising zinc or aluminium.

Further aspects and advantages of the invention will appear upon reading the following description, given by way of example and made with reference to the accompanying drawings, in which:

figure 1 is a schematic cross-sectional view of two metal sheets positioned according to a first embodiment of a method for butt-welding two metal sheets;

figure 2 is a schematic top view of the two metal sheets of figure 1 during a butt-welding step of the method according to the first embodiment;

fig. 3 is a schematic cross section of one of the two metal sheets of fig. 2, according to plane III-III in fig. 2, and;

figures 4 to 8 are schematic top views of two metal sheets during a butt-welding step of a method for butt-welding two metal sheets according to the second, third, fourth, fifth and sixth embodiments, respectively.

A method for butt laser welding two

metal plates

2, 4 according to a first embodiment of the present invention will be described with reference to fig. 1 to 3.

The method comprises the step of providing a first metal sheet 2 and a second metal sheet 4.

Each

metal sheet

2, 4 has two

main surfaces

6, 7, respectively, and a side surface 8 joining the two

main surfaces

6, 7.

The

main surfaces

6, 7 are an upper surface 6 and a lower surface 7. The terms "upper" and "lower" are intended to refer to axes perpendicular to said

main surfaces

6, 7.

The side surface 8 of each

metal plate

2, 4 extends, for example, perpendicularly to the

main surfaces

6, 7 of the

metal plates

2, 4. Alternatively, the side surface 8 of at least one of the

metal sheets

2, 4 does not extend perpendicular to the

main surfaces

6, 7. For example, the lateral surface 8 of at least one

metal sheet

2, 4 is inclined with respect to the

main surfaces

6, 7 of that

metal sheet

2, 4 and forms an angle different from 90 °, for example greater than or equal to 45 °, more particularly greater than or equal to 60 °, with one of the

main surfaces

6, 7.

The thicknesses of the first metal plate 2 and the second metal plate 4 are 0.15mm to 5mm, respectively. By "thickness of the metal plate" is meant the distance between the

main surfaces

6, 7 of the

metal plates

2, 4, taken perpendicular to said

main surfaces

6, 7.

Preferably, the first metal plate 2 and the second metal plate 4 each have a constant thickness. Alternatively, at least one of the first metal plate 2 and the second metal plate 4 has a variable thickness.

The first metal plate 2 and the second metal plate 4 have, for example, the same thickness. Alternatively, they have different thicknesses.

The first metal sheet 2 and/or the second metal sheet 4 comprise a steel substrate 9A.

The steel of the matrix 9A is more particularly a steel having a ferritic-pearlitic microstructure.

Preferably, the base body 9A is made of steel intended for heat treatment, more particularly press-hardenable steel, and for example manganese-boron steel (for example 22MnB5 type steel).

Alternatively, the microstructure of the steel matrix 9A comprises bainite and/or ferrite and/or retained austenite.

Depending on its desired thickness, the matrix may be obtained by hot rolling and/or by cold rolling followed by annealing, or by any other suitable method.

The

plates

2, 4 comprise, for example, on at least one main surface of the basic body 9A and, for example, on both main surfaces thereof, a precoat layer 9B comprising zinc or aluminium.

The method further comprises the step of positioning the first metal sheet 2 and the second metal sheet 4 for butt laser welding.

As shown in fig. 1, is positioned such that the side surfaces 8 of the first metal plate 2 and the second metal plate 4 face each other.

In the example shown in fig. 2, there is at least one area of the first metal plate 2 and the second metal plate 4 in which the side surfaces 8 facing each other are located spaced apart from each other. Alternatively, the first metal plate 2 and the second metal plate 4 are in contact, for example, over the entire area of their side surfaces 8. In another alternative, there is at least one area of the first metal plate 2 and the second metal plate 4 in which the side surfaces 8 are in contact with each other only over a part of their thickness.

The

main surfaces

6, 7 are positioned substantially parallel to each other. By "substantially parallel" it is meant that the

main surfaces

6, 7 define a first plane and a second plane, respectively, which define an angle between them of less than 1 °.

The positioning defines a median plane 10 perpendicular to the

main surfaces

6, 7 of the first metal sheet 2 and the second metal sheet 4. The intermediate plane 10 is in particular an intermediate plane between the side surfaces 8. The median plane 10 is preferably vertical.

The method then comprises the step of butt welding the first metal sheet 2 and the second metal sheet 4 in a welding direction. The welding direction extends in particular along the median plane 10. In fig. 2, an arrow 11 indicates a welding direction.

This step of butt-welding includes simultaneously emitting a first front laser beam 12, a second front laser beam 14, and a rear laser beam 16.

The

laser beams

12, 14, 16 are made of, for example, CO₂Laser, YAG-Nd laser, fiber laser, disk laser or direct diode laser. The

different laser beams

12, 14 and 16 may be generated by the same type of laser or by different types of lasers.

In one embodiment, the first and second

front laser beams

12, 14 and/or the back laser beam 16 are generated by a common laser head.

According to an alternative, each

laser beam

12, 14, 16 is generated by a dedicated laser head.

As a further alternative, two laser beams, for example two

front laser beams

12, 14, are generated by a common laser head and a third laser beam, for example a rear laser beam 16, is generated by a different laser head.

The welding direction results from the relative displacement between the first and

second metal plates

2, 4 and the

laser beams

12, 14 and 16.

The first front laser beam 12 is emitted along a first front emission axis E1. The first front emission axis E1 intersects one of the

main surfaces

6, 7 of the first metal plate 2. In the example shown in fig. 2, the main surface is the upper surface 6 of the first metal sheet 2. Alternatively, the main surface is the lower surface 7 of the first metal sheet 2.

For example, the first front emission axis E1 extends perpendicularly to the

main surfaces

6, 7 of the first metal plate 2.

As shown in fig. 2, the first front laser beam 12 generates a first front spot 18 at the intersection with said

main surfaces

6, 7 of the first metal sheet 2.

In particular, the first front laser beam 12 intersects said

main surfaces

6, 7 of the first metal sheet 2 over the entire cross section of the first front laser beam 12. Therefore, all the energy of the first front laser beam 12 is transmitted to the first metal plate 2.

The energy density of the first front laser beam 12 is greater than or equal to 10⁶W/cm². As a result, the first front laser beam 12 creates a first front keyway 19 (see fig. 3) in the first metal plate 2 at a first front spot 18. In fig. 3, the laser beam is not shown.

In this context, a keyway is a cavity that extends into the thickness of the metal plate and contains vaporized material resulting from the impact of a laser beam with the metal plate. The keyhole allows the energy of the associated laser beam to be transmitted directly into the metal plate.

As shown in fig. 3, during the butt-welding step, the first front laser beam 12 generates a first melt pool 13 at the location of the first front spot 18.

The first front keyway 19 is surrounded by the first weld puddle 13.

During the butt welding step, the pressure of the vapour contained in the first front keyway 19 prevents the molten material of the first melt pool 13 from collapsing into the cavity formed by the keyway.

In the example of fig. 3, first keyway 19 is shown as a cylindrical cavity extending perpendicular to

major surfaces

6, 8 for simplicity purposes only. In practice, the first keyway 19 may be inclined at an angle proportional to the welding speed relative to the normal to the

main surfaces

6, 8. Furthermore, the first keyway 19 can have a variable cross section over the entire plate thickness.

The second front laser beam 14 is emitted along a second front emission axis E2.

The second front emission axis E2 intersects one of the

main surfaces

6, 7 of the second metal plate 4. In the example shown in fig. 2, the main surface is the upper surface 6 of the second metal plate 4. Alternatively, the main surface is the lower surface 7 of the second metal plate 4.

For example, the second front emission axis E2 extends perpendicularly to the

main surfaces

6, 7 of the second metal plate 4.

The second front laser beam 14 generates a second front spot 20 at the intersection with said

main surface

6, 7 of the second metal sheet 4.

In the example shown in fig. 2, the first front spots 18 and the second front spots 20 are produced on the adjacent

main surfaces

6, 7 of the first metal sheet 2 and the second metal sheet 4.

"adjacent main surfaces" refer to one main surface of the first metal plate 2 and one main surface of the second metal plate 4 which are located on the same side of the

metal plates

2, 4 with respect to the emission direction of the laser beam. Thus, the first and second front spots 18, 20 are generated, for example, on the upper surface 6 of the first and

second metal plates

2, 4 or on the lower surface 7 of the first and

second metal plates

2, 4.

In particular, the second front laser beam 14 intersects said

main surfaces

6, 7 of the second metal sheet 4 over the entire transverse plane of the second front laser beam 14. Therefore, all the energy of the second front laser beam 14 is transmitted to the second metal plate 4.

The energy density of the second front laser beam 14 is greater than or equal to 10⁶W/cm². As a result, the second front laser beam 14 produces a second front keyhole (not shown) in the second metal plate 4 at the second front spot 20.

During the butt-welding step, the second front laser beam 14 generates a second melt pool at the location of the second front spot 20. The second melt pool surrounds the second front keyway.

For example, the volume of the first molten pool 13 is separated from the volume of the second molten pool, and at least one or each moment of the butt-welding step, the first molten pool 13 and the second molten pool are spaced apart from each other.

Alternatively, the volume of the first molten pool 13 is separated from the volume of the second molten pool, and at least one or each moment of the butt welding step, a solid phase region of the

metal sheets

2, 4 remains between the first molten pool 13 and the second molten pool.

Alternatively, at least one time during the butt welding step, the first and second melt pools join to form a single melt pool.

The maximum size of the first front spot 18 and/or the second front spot 20 is 50 μm to 250 μm. Thus, the energy of the

laser beams

12, 14 is transmitted to the

metal plates

2, 4 with very high efficiency. Furthermore, these dimensions allow the first and

second keyway

19, 19 to be produced in the first and

second metal plates

2, 4 even in the case of a relatively low power input of the first and

second laser beams

14, 16.

The first front spot 18 and/or the second front spot 20 have a gaussian or top-hat energy distribution and preferably have a circular profile.

The distance taken along the welding direction between the centers of the first and second

front laser beams

12, 14 is less than or equal to 5 mm.

In particular, in the method shown in fig. 2, the centers of the first and second front spots 18 and 20 are aligned in a direction perpendicular to the welding direction.

The respective centers of the first front spot 18 and the second front spot 20 are located at a distance of less than or equal to 2.5mm from the side surface 8 of the first metal plate 2 and the second metal plate 4, respectively.

In particular, in the method shown in fig. 2, the centers of the first front spot 18 and the second front spot 20 are located at equal distances from the median plane 10 between the first metal sheet 2 and the second metal sheet 4.

During butt welding, the first and second front spots 18, 20 are spaced apart from each other. In other words, the first front spot 18 and the second front spot 20 do not overlap during butt welding.

The rear laser beam 16 is emitted along a rear emission axis E3.

The rear laser beam 16 intersects the adjacent

main surfaces

6, 7 of the first metal plate 2 and the second metal plate 4. These adjacent

main surfaces

6, 7 which intersect the rear laser beam 16 are for example the two upper surfaces 6 of the first metal plate 2 and the second metal plate 4 or the two lower surfaces 7 of the first metal plate 2 and the second metal plate 4.

For example, the rear emission axis E3 extends perpendicularly to the

main surfaces

6, 7 of the first and

second metal plates

2, 4.

The back laser beam 16 generates a back spot 22 on said adjacent

main surfaces

6, 7.

In the example shown in fig. 2, the rear spot 22 is produced on the same

main surface

6, 7 as the main surface on which the first front spot 18 and the second front spot 20 are produced. Alternatively, the rear spot 22 is produced on a different

main surface

6, 7 than the main surface on which the first front spot 18 and the second front spot 20 are produced.

The energy density of the rear laser beam 16 is greater than or equal to 10⁶W/cm². As a result, the rear laser beam 16 produces a rear keyhole 23A (see fig. 3) in the first metal plate 2 and the second metal plate 4 at the rear spot 22.

As shown in fig. 3, during the butt-welding step, the rear laser beam 16 generates a rear weld pool 23B at the location of the rear spot 22. Rear melt pool 23B surrounds rear keyway 23A.

In one embodiment, the energy of back laser beam 16 is greater than the respective energies of first and second

front laser beams

14, 16. For example, the energy of back laser beam 16 is at least twice the respective energies of first and second

front laser beams

14, 16.

In the method according to the first embodiment, the back spot 22 has a gaussian or top-hat energy distribution.

In the example shown in fig. 2, it has a circular profile.

As shown, the rear spot 22 is centered on the middle plane 10 between the first metal plate 2 and the second metal plate 4.

The maximum size of the rear spot 22 is, for example, 200 μm to 1800 μm, preferably 600 μm to 1200 μm.

As shown in FIG. 2, the surface area of the back spot 22 is greater than the surface area of each of the first front spot 18 and the second front spot 20. The volume of the rear keyway 23A produced by the rear laser beam 16 is greater than the volume of the first front keyway and the second front keyway produced by the first front laser beam 12 and the second front laser beam 14, respectively. Furthermore, during butt welding, the volume of the rear melt pool 23B is greater than the volumes of the first and second melt pools generated by the first and second

front laser beams

12 and 14, respectively.

During butt welding, the first and second

front laser beams

12, 14 and the rear laser beam 16 have a relative movement with respect to the first and

second metal plates

2, 4 such that the first and second front spots 18, 20 and the rear spot 22 are displaced in the welding direction with respect to the first and

second metal plates

2, 4.

For example, the first and second

front laser beams

12 and 14 and the rear laser beam 16 are moved together while the first and

second metal plates

2 and 4 are fixed in place. Alternatively, the first and second

front laser beams

12 and 14 and the rear laser beam 16 are fixed in place, so that the first and

second metal plates

2 and 4 move together.

Laser welding produces a welded joint 24 at the junction of the two

metal sheets

2, 4. The welding direction is defined as the direction extending from the area in which the weld joint 24 has been created along the middle plane towards the area in which the

plates

2, 4 have not yet been joined by the weld joint 24.

The first and

second laser beams

12, 14 and the back laser beam 16 are arranged in such a way that the first and second front spots 18, 20 are located in front of the back spot 22.

In this context, "anterior" means anterior with respect to the welding direction. Thus, the rear spot 22 is located between the

front spots

18, 20 and the weld joint 24 along the welding direction. In other words, during the butt-welding step, a given area of the first and

second metal plates

2, 4 centered on the median plane 10 always intersects first the first and second front spots 18, 20 and then subsequently the rear spots 22.

During butt welding, the rear spot 22 is spaced apart from each of the first and second front spots 18, 20. In other words, during butt welding, the rear spot 22 does not overlap each of the first and second front spots 18 and 20.

During butt welding, rear keyway 23A is spaced from first front keyway 19 and second front keyway. In other words, the rear keyway 23A does not overlap each of the first and second front keyways during butt welding.

For example, the relative geometric arrangement of the keyway during butt welding may be monitored by visualizing the laser material interaction zone via a 2D vision sensor. With an infrared 2D vision sensor, a 2D temperature map of the interaction region can be generated. In particular, the keyhole, the melt pool and the solid phase region are clearly identified. Alternatively or additionally, the keyway shape may be visualized with a 2D pure vision sensor by illuminating the laser-material interaction region with light of a dedicated wavelength different from the welding laser and using a corresponding pass-band filter in front of the vision sensor.

The first and second

front laser beams

12, 14 and the rear laser beam 16 are configured in such a way that at each moment during the butt welding step a solid phase region 25 and/or a

liquid phase region

13, 23B of the

metal plates

2, 4 remains between the first front keyway 19 and the rear keyway 23A and between the second front keyway and the rear keyway 23A.

More particularly, the size of the

spots

18, 20, 22; the distance between the

spots

18, 20, 22; relative displacement speeds between the first and second

front laser beams

12, 14 and the rear laser beam 16 and the

metal plates

2, 4; and the power density of the light beams 12, 14, 16 are configured in such a way that at each moment during the butt welding step a solid phase region 25 and/or a

liquid phase region

13, 23B of the

metal plates

In the example shown in fig. 3, a solid phase region 25 and a

liquid phase region

13, 23B of the

metal plates

2, 4 remain between the first front keyway 19 and the rear keyway 23A and between the second front keyway and the rear keyway 23A. Alternatively, only the

liquid phase regions

13, 23B of the

metal plates

2, 4 remain between the first front keyway 19 and the rear keyway 23A and between the second front keyway and the rear keyway 23A.

In one embodiment, first and second

front laser beams

12, 14 and rear laser beam 16 are configured in such a way that at each time during the butt-welding step, the volume of the melt pool created by first and second

front laser beams

12, 14 is spaced apart from the volume of the melt pool created by rear laser beam 16. This embodiment reduces the variability of the weld and eases its numerical simulation compared to the case where a single weld pool is formed by the front and rear laser beams.

The center of the rear spot 22 extends at a distance of 0.5mm to 8mm taken in the welding direction from the center of the rearmost of the first and second front spots 18, 20. The distance is preferably 1mm to 5 mm.

The displacement speeds of the first and second

front laser beams

12, 14 and the rear laser beam 16 are preferably the same and are 2 m/min to 20 m/min, preferably 4 m/min to 12 m/min.

Examples of spot size, distance between spots, relative displacement speed of the spots on the

metal plates

2, 4, and power density of the light beam will now be described in detail.

According to a first example:

the first metal plate 2 and the second metal plate 4 each have a thickness of 1mm and are positioned such that they contact each other along the welding direction or such that they are spaced apart from each other by a distance of less than 80 μm;

the first front spot 18 and the second front spot 20 have a diameter equal to 150 μm and an energy of 500W, respectively, the first front spot 18 and the second front spot 20 being produced by a common laser welding head driven by a disc-shaped laser generator with a power of 1kW and a wavelength of 1 μm;

the rear spot 22 has a diameter equal to 600 μm and an energy of 4kW, the rear spot 22 being produced by a dedicated laser welding head driven by a YAG laser with a power of 4kW and a wavelength of 1 μm;

the

front spots

18, 20 and the rear spot 22 are arranged in an equilateral triangle, such that the side length of the equilateral triangle is 1.2mm, the rear spot 22 is centered on the middle plane and the

front spots

18, 20 are located at equal distances from the middle plane, and;

the welding speed was 16 m/min.

According to a second example:

the first front spot 18 and the second front spot 20 have a diameter equal to 150 μm and an energy of 500W, respectively, the first front spot 18 and the second front spot 20 being produced by a common laser welding head driven by a disc laser with a power of 1000W and a wavelength of 1 μm;

the

front spots

18, 20 are located at equal distances from the median plane and at a distance of 0.6mm, the rear spot 22 is centered on the median plane and located at a distance of 1.2mm behind the

front spots

18, 20, and;

the welding speed was 16 m/min.

In the method for butt laser welding two

metal plates

2, 4 according to the present invention, the first front keyway and the second front keyway have a high shape ratio, which is defined as the ratio of the height of the keyway to the diameter of the keyway. A keyway having a high aspect ratio absorbs the greatest amount of laser beam energy due to the multiple reflections of the laser beam in the keyway.

In particular, having a solid phase region 25 and/or a

liquid phase region

13, 23B between the keyway prevents the first front keyway and the second front keyway from opening into the rear keyway 23A. Such communication between the front keyway and the rear keyway will result in a substantial reduction in the respective aspect ratio of the front keyway and thus affect the energy efficiency of the butt welding step.

Furthermore, during butt welding, the first and second

front laser beams

12, 14 preheat the regions of the first and

second metal sheets

2, 4 under consideration before they are melted by the rear spot 22, respectively.

Such preheating is advantageous because it improves the energy efficiency of the butt weld. In addition, this also causes the rear molten pool 23B to be elongated. The extended molten pool 23B improves the geometry of the weld joint 24 and increases the speed limit of the butt weld.

Furthermore, in the case of a coating on the

steel sheets

2, 4, the first and second

front laser beams

12, 14 act on the coating in the following manner: a coating having a low vaporization temperature, for example a zinc-based coating, is vaporized by the action of the first and second

front laser beams

12, 14, while a coating having a high vaporization temperature, for example an aluminum-based coating (for example an aluminum-silicon coating), is pre-melted and partially incorporated into the melting zone.

The respective energy densities of the first front spot 18 and the second front spot 20 allow to very efficiently preheat the first metal plate 2 and the second metal plate 4. The efficiency is attributed to the first front keyway and the second front keyway having a high shape ratio.

Further, during butt welding, the first and second

front laser beams

12 and 14 tend to reduce the gap between the first and

second metal plates

2 and 4. More precisely, the gap width between the first metal sheet 2 and the second metal sheet 4 decreases due to the thermal expansion of the steel base 9A and due to the surface tension at the side surface 8 (which tends to bend the edge profile of the side surface 8 and move it closer to the mid-plane 10). This reduction in gap width results in an increase in the energy efficiency of the rear laser beam 16.

In an alternative of the method according to the first embodiment, the centers of the first front spot 18 and the second front spot 20 are arranged at a distance from each other along the welding direction.

In another alternative of the method according to the first embodiment, the method comprises providing a welding material, such as welding wire or welding powder, during the butt welding step.

The welding material is preferably provided to the back laser beam 16 by being interposed between the first front laser beam 12 and the second front laser beam 14. Alternatively, the added weld material may be provided laterally or at a location posterior to the posterior laser beam 16.

With the centers of the first and second front spots 18, 20 arranged at a distance from each other along the welding direction, it is facilitated to insert welding material between the first and second

front laser beams

12, 14.

In the case where the welding material is a welding wire, the welding wire is inserted between the first front laser beam 12 and the second front laser beam 14, for example, substantially parallel with respect to the median plane 10.

When inserted between the first front laser beam 12 and the second front laser beam 14, the welding material is preheated by the first front laser beam 12 and the second front laser beam 14. When the welding material is inserted between the first front laser beam 12 and the second front laser beam 14, less than 20% of the volume of the welding material is melted by these

laser beams

12, 14. Preferably, first and second

front laser beams

12 and 14 do not melt the welding material when the welding material is interposed between first and second

front laser beams

12 and 14.

In this alternative, the elongation of the rear weld pool 23B due to the preheating of the

metal sheets

2, 4 by the first and second front spots 18, 20 contributes to an improved mixing of the welding material with the material of the

metal sheets

2, 4.

Instead of the method according to the first embodiment, the centers of the first front spot 18 and the second front spot 20 are located at different distances from the intermediate plane 10.

In another alternative of the method according to the first embodiment, the center of the rear spot 22 is laterally offset with respect to the median plane 10 between the first metal plate 2 and the second metal plate 4. This alternative is particularly suitable when the

metal sheets

2, 4 have different thicknesses.

According to another alternative of the method according to the first embodiment, the posterior spot 22 has an elongated outer contour, such as an oblong (oblong) contour, an elliptical contour, a rectangular contour, a teardrop-shaped contour or an oval contour. The oblong, teardrop and oval profiles will be described in more detail with reference to fig. 5, 6 and 7, respectively.

A method for butt laser welding two

metal plates

2, 4 according to a second embodiment will be described with reference to fig. 4.

This method differs from the method according to the first embodiment in that the rear spot 22 is annular.

The energy of the rear laser beam 16 is mainly concentrated in the outer annular portion rather than the central portion thereof. Such a shape makes it easier to reach 10⁶W/cm²The energy density of (1).

In particular, "annular" means that the posterior spot 22 has an outer contour 26 and an inner contour 28. Inner contour 28 is substantially similar to outer contour 26.

In particular, the rear spot 22 is symmetrical with respect to a plane parallel to the median plane 10 between the two

metal plates

2, 4.

As shown in fig. 4, the rear spot 22 is circular. More specifically, the outer contour 26 and the inner contour 28 of the posterior spot 22 are circular.

The maximum outer dimension of the rear spot 22 is 200 μm to 1800 μm, preferably 600 μm to 1200 μm. In the method according to the second embodiment, the maximum outer dimension of the rear spot 22 corresponds to the diameter of its outer contour 26.

The ratio of the maximum outer dimension to the maximum inner dimension of the rear spot 22 is 1.2 to 3.2, preferably 1.3 to 2. In a second approach, the maximum inner dimension of the rear spot 22 corresponds to the diameter of its inner contour 28.

As shown in fig. 4, at least a portion of the inner contour 28 intersects the first metal plate 2 and the second metal plate 4, respectively.

The annular shape of the rear laser beam 16 allows the energy efficiency of the method to be increased, since the portion of the energy of the rear laser beam 16 wasted by passing through the gap between the side surfaces 8 is reduced.

A method for butt laser welding two

metal plates

2, 4 according to a third embodiment will be described with reference to fig. 5.

This method according to the third embodiment differs from the method according to the second embodiment in that the rear spot 22 has an elongated shape along an elongation direction parallel to the welding direction. In this case, the outer dimension of the rear spot 22 taken perpendicular to the welding direction is smaller than the outer dimension of the rear spot 22 taken parallel to the welding direction.

In the example shown in fig. 5, the rear spot 22 has an oblong shape. More precisely, in the case of a ring-shaped posterior spot 22, the outer 26 and inner 28 contours of the posterior spot 22 are oblong.

The elongated shape improves the flow of molten material behind the rear spot 22, which is less turbulent. Thus, the elongated shape improves the geometry of the weld joint 24 even further and increases the speed limit of the butt weld.

According to one alternative, the rear spot 22 has an elliptical shape. More precisely, in the case of a ring-shaped posterior spot 22, the outer contour 26 and the inner contour 28 of the posterior spot 22 are elliptical.

According to another alternative, the rear spot 22 has a rectangular outline. More precisely, in the case of a ring-shaped posterior spot 22, the outer contour 26 and the inner contour 28 of the posterior spot 22 are rectangular.

A method for butt laser welding two

metal plates

2, 4 according to a fourth embodiment will be described with reference to fig. 6.

The method according to the fourth embodiment differs from the method according to the third embodiment in that the posterior spot 22 has a tear-drop shape.

More specifically, in the case of a circular posterior patch 22, the outer and

inner contours

26, 28 of the posterior patch 22 are tear drop shaped.

More particularly, as can be seen in fig. 6, each of outer profile 26 and inner profile 28 has a pointed end 30 opposite a rounded edge 32.

As shown in fig. 6, the pointed end 30 is located behind the rounded edge 32 in the welding direction.

The pointed end 30 is centered on the median plane 10.

The tear-drop shape improves the flow of molten material even further behind the rear spot 22.

A method for butt laser welding two

metal plates

2, 4 according to a fifth embodiment will be described with reference to fig. 7.

The method according to the fifth embodiment differs from the method according to the third embodiment in that the rear spot 22 has an oval shape.

A method for butt laser welding two

metal plates

2, 4 according to a sixth embodiment will be described with reference to fig. 8.

The method according to the sixth embodiment is different from the method according to the third embodiment in that it further includes emitting a second rear laser beam 34 simultaneously with the step of emitting the first front laser beam 12, the second front laser beam 14, and the rear laser beam 16.

The second rear laser beam 34 is emitted along a second rear emission axis E4.

The second rear laser beam 34 intersects the adjacent

main surfaces

6, 7 of the first metal plate 2 and the second metal plate 4.

The second rear laser beam 34 generates a second rear spot 36 on said adjacent

main surfaces

6, 7.

In the example shown in fig. 8, the second rear spot 36 is produced on the same

main surface

6, 7 as the main surface on which the rear spot 22 is produced. Alternatively, the second rear spot 36 is produced on a different

main surface

6, 7 than the main surface on which the rear spot 22 is produced.

The energy density of the second rear laser beam 34 is preferably greater than or equal to 10⁶W/cm². As a result, the second rear laser beam 34 creates a second rear keyhole in the first metal plate 2 and the second metal plate 4 at a second rear spot 36.

Alternatively, the energy density of second post laser beam 34 is less than 10⁶W/cm²Thus, non-keyhole heating is generated, thereby enhancing the smoothness of the weld joint 24.

The second rear spot 36 is symmetrical with respect to a plane parallel to the median plane 10 between the two

metal sheets

2, 4.

In particular, the second rear spot 36 is circular. More specifically, the outer contour 38 and the inner contour 40 of the second rear spot 36 are circular.

The maximum outer dimension of the rear spot 22 is larger than the maximum outer dimension of the second rear spot 36.

The maximum size of the second rear spots 36 is, for example, 100 μm to 1300 μm, preferably 300 μm to 1000 μm.

In the example of fig. 8, the second rear spot 36 is centered on the mid-plane 10.

During butt welding, the second rear laser beam 34 is configured in such a way that the second rear spot 36 is located behind the rear spot 22 in the welding direction. The second rear spot 36 is located in particular between the rear spot 22 and the weld joint 24 along the welding direction.

During the butt-welding step, the second rear laser beam 34 generates a second rear melt pool, which is generated in particular at the location of the second rear spot 36.

For example, the rear melt pool 23B and the second rear melt pool join. Alternatively, the volume of the rear molten pool 23B is separated from the volume of the second rear molten pool, and at each moment of the butt welding step, a solid phase region of the

metal sheets

2, 4 remains between the rear molten pool 23B and the second rear molten pool.

In the example shown in fig. 8, the second rear spot 36 is annular. The second rear spot 36 has in particular an outer contour 38 and an inner contour 40. The inner contour 40 of the second rear spot 36 is significantly similar to the outer contour 38 of the second rear spot 36.

According to one alternative, the second rear spot 36 has a gaussian or top-hat energy distribution.

According to another alternative, the second rear spot 36 has an elongated shape, such as an oblong shape, an oval shape, a rectangular shape, or a teardrop shape.

In another alternative, the center of the second rear spot 36 is laterally offset with respect to the median plane 10 between the first metal plate 2 and the second metal plate 4.

Due to the above disclosed features, the butt welding method according to the present invention makes it possible to weld the

metal plates

2, 4 with good energy efficiency even if the

metal plates

2, 4 are positioned with a significant gap therebetween.

Claims

1. A method for laser welding two metal sheets (2, 4), the method comprising the steps of:

-providing a first metal sheet (2) and a second metal sheet (4), each having two main surfaces (6, 7) and a side surface (8) joining the two main surfaces (6, 7), respectively;

-positioning the first metal sheet (2) and the second metal sheet (4) so that the side surfaces (8) thereof face each other, the positioning of the first metal sheet (2) and the second metal sheet (4) defining a median plane (10) perpendicular to the main surfaces (6, 7) of the first metal sheet (2) and the second metal sheet (4); and

-butt welding the first metal sheet (2) and the second metal sheet (4) along a welding direction, the butt welding step comprising simultaneous emission:

-a first front laser beam (12) along a first front emission axis (E1), the first front emission axis (E1) intersecting one of the main surfaces (6, 7) of the first metal sheet (2), the first front laser beam (12) generating a first front spot (18) at the intersection with the main surface of the first metal sheet (2), the first front laser beam (12) having an energy density greater than or equal to 10⁶W/cm²-the first front laser beam (12) creates a first front keyway (19) in the first metal plate (2) at the first front spot (18);

-a second front laser beam (14) along a second front emission axis (E2), the second front emission axis (E2) intersecting one of the main surfaces (6, 7) of the second metal sheet, the second front laser beam (14) generating a second front spot (20) at the intersection with the main surface of the second metal sheet, the energy density of the second front laser beam (14) being greater than or equal to 10⁶W/cm²-the second front laser beam (14) creates a second front keyway in the second metal plate (4) at the second front spot (20);

the respective centers of the first front spot (18) and the second front spot (20) are located at a distance of less than or equal to 2.5mm from the side surface (8) of the first metal plate (2) and the second metal plate (4), respectively, and the distance taken in the welding direction between the centers of the first front laser beam (12) and the second front laser beam (14) is less than or equal to 5 mm; and

-a rear laser beam (16), said rear laser beam (16) intersecting adjacent main surfaces (6, 7) of said first metal sheet (2) and said second metal sheet (4) and creating a rear spot (22) thereon, said rear laser beam (16) having an energy density greater than or equal to 10⁶W/cm²-the rear spot (22) has a surface area larger than the respective surface areas of the first front spot (18) and the second front spot (20), -the rear laser beam (16) creates a rear keyway (23A) in the first metal plate (2) and the second metal plate (4) at the rear spot (22);

the first and second front laser beams (12, 14) and the rear laser beam (16) are configured in such a way that:

-the first front spot (18) and the second front spot (20) are located in front of the rear spot (22); and such that:

-a solid phase region (25) and/or a liquid phase region (13, 23B) of the metal plates (2, 4) remains between the first front keyway (19) and the rear keyway (23A) and between the second front keyway and the rear keyway (23A) at each time.

2. Method according to claim 1, wherein at each instant of the butt-welding step, the volume of the weld pool generated by the first front laser beam (12) and the second front laser beam (14) is separated from the volume of the weld pool generated by the rear laser beam (16).

3. The method according to claim 1, wherein the first front spot (18) and the second front spot (20) have a circular contour, the maximum dimension of the first front spot (18) and/or the second front spot (20) being between 50 μ ι η and 250 μ ι η.

4. The method according to any one of claims 1 to 3, wherein the posterior spot (22) has a circular outer contour, the diameter of the posterior spot (22) being from 200 μm to 1800 μm.

5. The method according to claim 4, wherein the diameter of the circular outer contour of the rear spot (22) is 600 μm to 1200 μm.

6. A method according to any one of claims 1 to 3, wherein the rear spot (22) has an elongated shape along an elongation direction parallel to the welding direction; wherein the outer dimension of the rear spot (22) taken perpendicular to the welding direction is smaller than the outer dimension of the rear spot (22) taken parallel to the welding direction;

wherein the rear spot (22) has an outer dimension taken parallel to the welding direction of between 200 μm and 1800 μm.

7. Method according to claim 6, wherein the rear spot (22) has an outer dimension, taken parallel to the welding direction, of between 600 μm and 1200 μm.

8. A method according to any one of claims 1 to 3, wherein the first metal sheet (2) and the second metal sheet (4) each have a thickness of 0.15mm to 5 mm.

9. A method according to any one of claims 1 to 3, wherein the centres of the first front spot (18) and the second front spot (20) are located at equal distances from the median plane (10) between the first metal plate (2) and the second metal plate (4).

10. Method according to any one of claims 1 to 3, wherein the centers of the first front spot (18) and the second front spot (20) are aligned along a direction perpendicular to the welding direction.

11. Method according to any one of claims 1 to 3, wherein the centers of the first front spot (18) and the second front spot (20) are arranged at a distance from each other along the welding direction.

12. A method according to any one of claims 1 to 3, wherein the rear spot (22) is centred on the intermediate plane (10) between the first metal plate (2) and the second metal plate (4).

13. A method according to any one of claims 1 to 3, wherein the centre of the rear spot (22) is laterally offset with respect to the mid-plane (10) between the first metal plate (2) and the second metal plate (4).

14. A method according to any one of claims 1 to 3, wherein the centre of the rear spot (22) extends at a distance of 0.5mm to 8mm taken along the welding direction from the centre of the rearmost of the first front spot (18) and the second front spot (20).

15. The method according to claim 14, wherein the center of the rear spot (22) extends at a distance of 1mm to 5mm taken along the welding direction from the center of the rearmost of the first and second front spots (18, 20).

16. The method according to any one of claims 1 to 3, wherein the first anterior spot (18) and/or the second anterior spot has a Gaussian or top-hat energy distribution.

17. The method according to any one of claims 1 to 3, wherein the posterior spot (22) has a Gaussian or top-hat energy distribution.

18. The method according to any one of claims 1 to 3, wherein the rear spot (22) is annular.

19. Method according to claim 18, wherein the outer dimension of the rear spot (22) taken perpendicular to the welding direction is smaller than the outer dimension of the rear spot (22) taken parallel to the welding direction.

20. Method according to claim 18, wherein the rear spot (22) is symmetrical with respect to a plane parallel to the median plane (10) between the two metal plates (2, 4).

21. The method according to claim 19, wherein the maximum outer dimension of the rear spot (22) is 200 μ ι η to 1800 μ ι η.

22. The method according to claim 21, wherein the maximum outer dimension of the rear spot (22) is 600 to 1200 μ ι η.

23. The method according to claim 19, wherein the ratio of the maximum outer dimension to the maximum inner dimension of the rear spot (22) is 1.2 to 3.2.

24. The method according to claim 23, wherein the ratio of the maximum outer dimension to the maximum inner dimension of the rear spot (22) is 1.3 to 2.

25. Method according to any one of claims 1 to 3, wherein the rear spot (22) has a circular profile or an elongated shape along an elongation direction parallel to the welding direction.

26. The method of any of claims 1-3, wherein the method further comprises: simultaneously with the steps of emitting the first front laser beam (12), the second front laser beam (14) and the back laser beam (16), emitting a second back laser beam (34), the second back laser beam (34) intersecting adjacent major surfaces (6, 7) of the first metal sheet (2) and the second metal sheet (4) and creating a second back spot 36 thereon, the second back laser beam (34) being configured in such a way that the second back spot (36) is located behind the back spot (22).

27. The method according to claim 26, wherein the second rear spot (36) is annular.

28. The method of claim 26, wherein the second back spot (36) has a gaussian or top-hat energy distribution.

29. The method according to claim 26, wherein the maximum outer dimension of the rear spot (22) is larger than the maximum outer dimension of the second rear spot (36).

30. The method of any of claims 1-3, wherein the method further comprises providing a weld material during the butt welding step.

31. The method of claim 30, wherein the welding material is welding wire or welding powder.

32. A method according to any one of claims 1 to 3, wherein the first metal sheet (2)/or the second metal sheet (4) comprises a steel substrate (9A) having a zinc alloy or aluminium alloy pre-coating (9B) on at least one of its main surfaces (6, 7).

33. Method according to any one of claims 1 to 3, wherein the first front laser beam (12) and/or the second front laser beam (14) and/or the rear laser beam (16) are generated by a common laser head

34. A method according to any one of claims 1 to 3 wherein each laser beam is generated by a dedicated laser head.

35. A method according to any one of claims 1 to 3, wherein the steel substrate (9A) of at least one of the first metal sheet (2) or the second metal sheet (4) is a press hardenable steel.

36. A method according to any one of claims 1 to 3, wherein at least one of the first metal sheet (2) or the second metal sheet (4) comprises a zinc-or aluminium-containing pre-coating.